1. Field of the Invention
The present invention relates to a relaying method for transformer protection. More specifically, the present invention relates to a relaying method using the ratio of induced voltages or the ratio of flux linkage increments.
2. Description of the Related Art
Current differential relays with harmonic constraints are widely used for power transformers. These relays determines a fault if a differential current between the primary and secondary windings is larger than a threshold value. The relays may malfunction in case of magnetic inrush or overexcitation. In order to prevent malfunction they use the second harmonic, third harmonic, or the total harmonic component of the current.
However, because of increase of operating voltages and length of transmission line, the differential currents during internal transformer faults can contain large harmonic components. Therefore, the security of the differential relays using harmonic restraints is a matter of concern. Also, these relays are inherently insensitive to turn-to-turn faults.
Recently, some algorithms use electromagnetic equations of transformers.
One of them is based on an equivalent circuit composed of inverse inductance. Another one is a flux-restrained current-differential relay. Both of them use winding currents those are practically unavailable in some situations such as a three-phase wye-delta transformer. Some elements used in the first method are nonlinear depending on the operating condition and thus it is very difficult to determine their values. In case of the second method, the winding resistance is neglected. But, practically it cannot be neglected. Thus, if there exits a winding resistance, large errors are contained.
Thus, the invention intends to overcome the above-mentioned problems and the purpose of the invention is to provide a protective relaying method for power transformers. In other words, the ratio of primary and secondary flux linkage increments is the same as the turn ratio in case of magnetic inrush and overexcitation, while the ratio is different from the turn ratio in case of internal winding faults. Thus, this invention provides a protective relaying method for power transformers; the method calculates the ratio of primary and secondary flux linkage increments from the primary and secondary voltages and currents and then compares the ratio with the turn ratio to discriminate magnetic inrush and internal winding faults.
And, another purpose of the invention is to provide a protective relaying method for power transformers using the ratio of primary and secondary induced voltages. In other words, the ratio of induced voltages is the same as the turn ratio in case of magnetic inrush and overexcitation, while the ratio is different from the turn ratio in case of internal winding faults. Thus, this invention provides a protective relaying method for power transformers; the method calculates the ratio of primary and secondary induced voltages from the primary and secondary voltages and currents and then compares the ratio with the turn ratio to discriminate magnetic inrush and internal winding faults.
According to a feature of the present invention to achieve the above object, the protective relaying method for power transformers with one or more phases includes the first step of obtaining primary and secondary voltages and currents of said transformer; the second step of calculating induced voltages, induced voltage differences, ratio of primary and secondary induced voltages, or ratio of primary and secondary induced voltage differences from said currents and said voltages; the third step of calculating at least one predetermined decision parameter derived from at least one predetermined equation; the fourth step of deciding whether an internal winding fault occurs by comparing said decision parameter to said induced voltages, said induced voltage differences, said ratio of primary and secondary induced voltages, or said ratio of primary and secondary induced voltage differences.
According to another feature of the present invention to achieve the above object, the protective relaying method for power transformers with one or more phases includes the first step of obtaining primary and secondary voltages and currents of said transformer; the second step of calculating flux linkage increments, flux linkage difference increments, the ratio of primary and secondary flux linkage increments, or ratio of primary and secondary flux linkage difference increments from said currents and said voltages; the third step of calculating at least one predetermined decision parameter derived from at least one predetermined equation; and the fourth step of deciding whether an internal winding fault occurs by comparing said decision parameter to said flux linkage increments, said flux linkage difference increments, said the ratio of primary and secondary flux linkage increments, or said ratio of primary and secondary flux linkage difference increments.